The present invention relates to a method of and apparatus for controlling the air-fuel ratio of an air-fuel mixture which is to be supplied to an internal combustion engine. More particularly, the invention is concerned with an air-fuel ratio controlling method and apparatus in which the mode of air-fuel ratio control is switched selectively in accordance with the state of the engine operation between a feedback control mode in which the air-fuel ratio is controlled in conformity with the stoichiometric one and a feed-forward control mode referred to as "lean control" in which the control is made to maintain an air-fuel ratio greater than the stoichiometric value, i.e. to maintain a mixture leaner than the stoichiometric one.
Generally, in automotive engines equipped with an exhaust gas scrubber of ternary catalyst type, it is necessary to effect the air-fuel ratio control such that the air-fuel ratio, which is directly related to the condition of combustion in the engine, is always maintained around the stoichiometric level, in order to keep the exhaust emissions clean.
To cope with this demand, a feedback control method has been proposed and used in which the oxygen content in the exhaust gases is detected by an O.sub.2 sensor as an index of the air-fuel ratio of the mixture, and the air-fuel ratio control is conducted in accordance with the output from the O.sub.2 sensor such that the air-fuel ratio coincides with the stoichiometric ratio.
When the engine is operating under comparatively light load, it is possible to decrease the rate of fuel consumption by maintaining the air-fuel ratio at the leaner side of the stoichiometric value without being accompanied by substantial degradation of the exhaust emissions because, under the light load, the rate of generation of nitrogen oxides is sufficiently small. Under these circumstances, an automotive engine has been proposed in which the control operation mode is selectively switched between the feedback control mode for maintaining the air-fuel ratio at the stoichiometric level and the lean control mode for maintaining the mixture at the leaner side of the stoichiometric one through a feed-forward control, thereby to minimize the fuel consumption.
The lean control, however, is an open loop control so that, if the lean control is conducted during idling in which the combustion is unstable, the engine operation may become unstable and, in the worst case, the engine may be stalled.